Refuse what a node cannot publish, and stop timing out work that is fine

Four things the python SDK turned up, each fixed where every client sees it.

A key no port declares is now an error rather than a silent drop, on the
return, the yield and the emit alike — the contract the docs already stated.
The SDK reads literal yields at sync time, so a typo fails before anything
runs, and an emission of one fails the call rather than being logged where
nobody looks.

NaN and infinity are refused at the port. JSON cannot spell either, so one
that travelled came back as a 500, a socket frame that stopped the canvas, or
a metric batch the database dropped whole.

An artifact input takes `@run:<id>.<output>` or a bare digest, resolved on the
engine — so the CLI, the run dialog and a python caller mean the same thing,
and a sweep can pass one at all.

Node timeouts are off by default. The clock measured silence, which a training
node is full of, and remote workers had already stopped enforcing it — their
heartbeat reset it. Now a heartbeat proves the agent rather than the node,
ninety seconds of nothing fails the call either way, and the engine touches
work it is still running so a long node is not redelivered at sixty seconds.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019V5bsYGNxcgPs4xXmTPx69

docs/code/cli.md

@@ -182,6 +182,20 @@ the numbers the run reports as they arrive:
 Ctrl-C while either is waiting cancels the run on the engine rather than only
 stopping the watching, and exits 130.
 
+An input declared as an `artifact` takes the file a previous run produced,
+named rather than typed out:
+
+```sh
+fluksio run evaluate --dataset @run:1758042000123-9f2ab41c.dataset
+fluksio run evaluate --dataset sha256:6dd1f0…
+```
+
+`@run:<id>.<output>` is what that run's output was, and a bare digest is the
+content itself; the engine resolves either into the reference. Passing the
+whole reference as JSON still works and is what a script that already holds one
+does — which is the same thing `flow.submit(dataset=run.result["dataset"])`
+does from Python.
+
 `--no-sync` runs what is already on the engine. Worth it in a tight loop where
 you know nothing changed, since syncing retires the workers and the next call
 pays its imports again. A directory that declares no flows syncs nothing and